The invention concerns an apparatus for mounting semiconductor chips on a substrate.
Such automatic assembly machines are generally known as Die Bonders. On mounting the semiconductor chips, a transport arrangement feeds the substrate in steps to a dispensing station where adhesive is applied and then to a bonding station where the next semiconductor chip is placed. Transport arrangements have become known from the U.S. Pat. No. 5,163,222 and the Swiss patent CH 689 188 with which the alignment of the substrate at right angles to the transport direction takes place mechanically in that the substrate is pulled against a stop. A disadvantage with this solution is that certain substrates bend when they are pulled against the stop. On the other hand, automatic assembly machines are available on the market with which the position of the substrate is determined and corrected immediately before placing the next semiconductor chip by means of a camera and electronic image processing. Such a solution is known from European patent application EP 877 544. This solution has two considerable disadvantages: It is time-consuming at the cost of the cycle time and is expensive. High placement accuracy and short cycle times however are demanded from automatic assembly machines.
A leadframe work station with a movable indexing head for the transport of the leadframe is known from U.S. Pat. No. 5,238,174. It is also known, that a leadframe may have a curved edge. The indexing head comprises a sensor, with which the position of the curved edge is detected before grasping and transporting the leadframe. As sensor a light barrier is foreseen. Positional errors due to the curvature of the edge of the leadframe can thus be partially compensated for. However after grasping, the leadframe is mechanically aligned at the indexing head. The indexing head transports the leadframe to the work station, where it is clamped by a downholder system while it is worked on. The placement accuracy achieves at best about 25 xcexcm. In order to achieve the accuracy which is necessary for example when wiring a semiconductor chip the leadframe is measured by means of a camera after clamping by the downholder system.
The object of the invention is to improve the placement accuracy of the mounted semiconductor chips at short cycle times.
The invention suggests measuring the position of the longitudinal edge of the leadframe at right angles to the transport direction at the height of the bonding station and then to carry out a corrective movement with the substrate so that the substrate position is presented with positional accuracy at the right location for bonding. An optical sensor with two light barriers arranged side by side is suggested for this measurement.